The present disclosure relates to electronic circuits, and more particularly to voltage controlled oscillators (VCOs).
A voltage controlled oscillator (VCO) is an electronic oscillator whose oscillation frequency is controlled by a voltage input. The input voltage may determine the instantaneous oscillation frequency. A VCO may also be connected to a phase-locked loop (PLL) circuit. In general, oscillators can be designed with a very large frequency range. However, oscillator gain tends to be nonlinear across the whole frequency range. For example, an LC VCO (in which L represents an inductor and C represents a capacitor) could have up to 10× change in VCO gain over a 3× frequency range. Similar issues may arise in ring oscillators. VCO gain may also be sensitive to variations in process, voltage and temperature (PVT). It should be noted that wide variations in VCO gain may lead to variations in PLL bandwidth which may result in large jitter variations over PLL range.
Techniques are known in the art to compensate the nonlinearities in the VCO gain, such as gain estimation and correction for digital PLLs (also known as Veyron PLL), matching VCO gain variations with charge pump current variations (e.g., in replica biased ring oscillators), and the like. However, these techniques may rely on correction of the nonlinearities in the VCO output in the PLL. It is therefore desirable to implement VCOs with linear gain over a very wide tuning range.